Multi-layered silver halide grains are known, such as those described in JP-A-60-143331, JP-A-62-196644, JP-A-61-112142, etc. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) In JP-A-62-123445, disclosed are multi-layered silver halide tabular grains having an aspect ratio, which is represented by the ratio of the circle-corresponding diameter of the major face of the grain to the thickness of the grain, of 1 or more. In these references, however, there is no disclosure relating to multi-layered tabular grains having {100} face as a major face.
The known, multi-layered silver iodobromide grains have high sensitivity and high resistance to pressure fog but have lower solubility than silver chloride grains. Therefore, though having high sensitivity, these are not suitable for photographic materials to be processed rapidly. When photographic materials having such silver iodobromide grains are processed, iodide ions and bromide ions accumulate in the developer being used thereby lowering the activity of the developer and retarding the development of the materials. In addition, the fixation of silver iodobromide emulsions progresses slowly and therefore the emulsions are not applicable to rapid processing.
There are many references relating to silver halide tabular grains having a high silver chloride content. As references relating to silver halide tabular grains having a major face of {111}, for example, mentioned are JP-B-64-8326, JP-B-64-8325, JP-B-64-8324 (the term "JP-B" as referred to herein means an "examined Japanese patent publication), JP-A-1-250943, JP-B-3-14328, JP-B-4-81782, JP-B-5-40298, JP-B-5-39459, JP-B-5-12696, JP-A-63-213836, JP-A-63-218938, JP-A-63-281149, and JP-A62-218959.
As references relating to silver halide tabular grains having a major face of {100}, mentioned are JP-A-5-204073 (corresponding to U.S. Pat No. 5,292,632), JP-A-51-88017 (corresponding to U.S. Pat No. 4,063,951), JP-A-63-24238 (corresponding to U.S. Pat No. 4,777,125), etc.
In JP-A-5-281640, referred to are core-shell type multi-layered grains. However, there is no reference relating to selenium and/or tellurium-sensitized emulsions of multi-layered or two-layered high silver chloride tabular grains having a major face of {100}.
It is a known that, in the crystal of a silver chloride grain, {100} face is more stable crystal habit than {111} face and the former is advantageous for adsorption of dye thereonto, etc. Therefore, it is easy to obtain silver chloride grains having high sensitivity. However, silver chloride grains having a uniform structure are often fogged when they are chemically sensitized. In addition, since the uniform silver chloride grains are not specifically constructed in such a way that the electric separation of electrons and positive holes to be formed in the grains when the grains have absorbed light is accelerated, the formation of latent images in or on the grains is often inefficient.
Moreover, silver chloride grains having elevated sensitivity are easily fogged under pressure. Therefore, it has heretofore been impossible to realize silver chloride grains having elevated sensitivity and elevated resistance to pressure fog.
We, the present inventors have found that, when the outermost layer of multi-layered silver halide grains is made to have a largest Br content rate, then the adsorption of dye onto the grains is enhanced to the same degree as that onto pure silver bromide grains. In addition, we also have found that such multi-layered silver halide grains where the outermost layer is made to have a high Br content rate are much more preferably used than pure silver bromide grains in photographic materials which are processable rapidly while needing reduced amounts of replenishers to the processing solutions being used for processing them.
In the present specification, "Br content rate" means a Br mol rate based on a silver halide composition constituting a region (layer) in a silver halide grain. For example, the "Br content rate" is "y" when the silver halide composition is AgIxBryClz and x+y+z=1.
When such multi-layered silver halide grains having the highest Br content rate region on the surfaces of the grains are exposed to light, positive holes generated by the exposure are gathered in the region and are forcedly separated from electrons while the rebinding of the positive holes and the electrons is inhibited. Accordingly, the formation of latent images on the grains is enhanced.
The existence of the high Br region content rate on the surface of the grain is equal to the introduction of the gap of halide composition and also the introduction of crystal defects (dislocation, etc.) inside of the grain, and it is well known that the introduction of these has the effect to reduce pressure fog. We, the present inventors have also found, as a result of our assiduous studies, that high silver chloride tabular grains having a major face of {100} outstandingly exhibit this effect.
In addition, we, the present inventor have also found that, when the formation of the shell around the core in producing core-shell type silver halide grains is conducted by conventional ion implantation under too high super-saturated conditions, it often detracts from the anisotropic growth of the growing grains with the result that the thus-grown grains defectively become thick.